1. Field of the Invention
The present invention relates to image forming apparatus, such as a copier and a printer. More particularly, the present invention relates to image forming apparatus in which density correction single-color and multi-color is performed.
2. Description of the Related Art
There is image forming apparatus which includes a plurality of photoreceptors and an intermediate transfer member. Each photoreceptor is exposed and carries a toner image. The intermediate transfer member functions as an image bearing member which carries the toner image transferred from each photoreceptor. In this configuration, it is effective to adjust a condition under which an image is formed in accordance with a detection result after detecting a toner image for adjustment on an intermediate transfer member for adjusting density of the image. If the toner image for adjustment is made to overlap a toner image for adjustment of another color, toner is not able to be detected for each color. Then, the toner image for adjustment is formed so as not to overlap the toner image of another color on the intermediate transfer member. That is, when passing a downstream photoreceptor (“second photoreceptor”) after being transferred to the intermediate transfer member, a toner image for adjustment formed on an upstream photoreceptor (“first photoreceptor”) faces an area of the second photoreceptor in which no toner image is formed, i.e., an area which has not been exposed.
If a color image is to be formed on a recording material, a toner image is made to overlap a toner image of another color to express secondary color. That is, when passing the downstream photoreceptor (“second photoreceptor”) after being transferred to the intermediate transfer member, the toner image formed on the upstream photoreceptor (“first photoreceptor”) faces an area of the second photoreceptor in which the toner image is formed, i.e., an area which has been exposed.
However, electrical potential of the exposed area differs from the potential of the unexposed area. Therefore, an amount of toner image which is retransferred from an intermediate transfer belt to the photoreceptor when the toner image passes the photoreceptor is changed depending on whether the toner image faces the exposed area or faces the unexposed area. Then, in Japanese Patent Laid-Open No. 2001-166558, an apparatus is described in which, when a patch conveyed from the upstream passes a downstream transfer unit, a downstream transfer electric field is weakened so that the amount of a toner image to be retransferred in the downstream transfer unit is reduced, and thereafter, density of the patch on which the toner image is transferred is detected and a developing condition of the toner image to be formed on the upstream is changed.
However, in the apparatus described above, if the toner image for adjustment is detected in a condition in which colors do not overlap each other, the toner image for adjustment formed on the upstream photoreceptor is detected in a condition different from a condition in which a color image is formed on the recording material due to the amount of retransferred toner image on the downstream photoreceptor. In that case, since detection of the toner image for adjustment is not performed in a condition in which color image formation is reproduced, there is a possibility that density adjustment in color image formation is not properly performed using the toner image for adjustment. Although the foregoing description is related to a configuration in which a plurality of photoreceptors are in contact with the intermediate transfer member as an image bearing member which carries a toner image, the same problem may exist in a configuration in which a plurality of photoreceptors are in contact with a recording material conveyance member which conveys a recording material.